Conventionally, there have been known halogen-containing, flame retardant copolymerized polycarbonates disclosed in Japanese Patent Publication Nos. 40715/1971 and 24660/1972 and Japanese Patent Application Laid-Open Nos. 123294/1976, 136796/1976, 140597/1977, 50065/1979, 99226/1981 and the like. However, these polycarbonates have been found not to have satisfactory properties for an industrial use, with respect to flame retardancy, mechanical strength and melt properties suitable for blow molding.
It has been known that branched polycarbonates with the improved flame retardancy are provided according to a method in which alkali metal salts of inorganic mineral acid, organic carboxylic aid or sulfonic acid are added thereto (as disclosed in Japanese Patent Publication No. 51497/1985) and a method in which flame retardant monomers, for example tetrabromobisphenol A, are copolymerized therewith (as disclosed in Japanese Patent Publication No. 12132/1980). However, the mechanical strength achieved by the former method has not been sufficiently high because of the contamination associated with the additives, while that of the latter method has been improved due to a branching agent contained therein but still not good enough for use in the industrial production.
Thus, the present inventors have made extensive studies with a view to developing a novel polycarbonate which is excellent in flame retardancy, mechanical properties and melt properties suitable for blow molding.